Satellite communication systems typically employ large aperture antennas and high power transmitters for establishing an uplink to the satellite. Recently, however, very small aperture antenna ground terminals, referred to as remote ground terminals, have been developed for data transmission at low rates. In such systems, the remote ground terminals are utilized for communicating via a satellite from a remote location to a central hub station. The central hub station communicates with multiple remote ground terminals, and has a significantly larger antenna, as well as a significantly larger power output capability than any of the remote ground terminals.
Typically, the remote ground terminals comprise a small aperture directional antenna for receiving and transmitting signals to a satellite, an outdoor unit (xe2x80x9cODUxe2x80x9d) mounted proximate the antenna for generating a modulated carrier signal to be transmitted to the central hub station and for receiving signals from the central hub station, and an indoor unit (xe2x80x9cIDUxe2x80x9d) which demodulates the received signals and which also operates as an interface between a specific user""s communication equipment and the outdoor unit.
Because the viability of the remote ground terminal concept increases as the cost for providing the remote ground terminal at the remote location decreases, it is necessary to decrease the cost of the remote ground terminal as much as possible. Furthermore, in order to decrease the operational costs, it is also necessary to decrease the size, weight and overall complexity of the remote ground terminal. Remote terminals with antenna diameters smaller than approximately 2 meters are often referred to as very small aperture terminals, or VSATs.
VSAT technology has followed a steady progression of cost and size reduction since the concept was first introduced in the early 1980s. Some of the earliest VSATs used a transmitter design topology known as xe2x80x9cdirect modulationxe2x80x9d in which the RF carrier is modulated directly within the ODU. This concept provided several advantages over the previous earth station design practice in which a modulated IF carrier is fed to the ODU from the IDU and upconverted. Some drawbacks exist with direct modulation however, in that the VSAT""s synthesizer (typically comprised of two parts: a xe2x80x9cwideband loopxe2x80x9d and a xe2x80x9cnarrowband loopxe2x80x9d) must be located in the outdoor equipment which is subject to more environmental extremes (temperature, vibration, etc.) than the indoor equipment. Also, due to the large number of circuits in the ODU, AC power is typically required at the outdoor equipment. A further disadvantage of this approach is that the modulating data (the transmitter""s input signal) and the synthesizer control signals must be conveyed to the ODU via some means. This was typically done with an expensive multi-pair cable running between the IDU and ODU (the intra-facility link, or IFL), although other multiplexing schemes have been used.
Later VSATs reduced the size and component count in the ODU by relocating the synthesizer to the IDU. In this design, the ODU used a phase-locked-loop (PLL) to develop an RF local oscillator (LO) signal. This LO signal was used to upconvert the IF signal fed from the IDU. The advantages of a smaller ODU package are readily apparent, but a wideband (typically 500 MHz) IFL cable between the IDU and the ODU is required. This cable is costly and limits the maximum distance between the IDU and ODU. Also, control and stabilization of the output power level is more complex than in the previous example.
A third type of known VSAT design falls somewhere between the previous two in concept. In this third approach, the synthesizer is split with the narrowband loop in the IDU and the wideband loop in the ODU. The modulation is performed in the IDU to minimize the amount of equipment located outdoors. The cost of the IFL cable can be reduced because it is not necessary to carry a wide bandwidth transmit signal to the ODU. A lower cost xe2x80x9cnarrowbandxe2x80x9d IFL cable can be used. There are several drawbacks to this approach. Because the wideband synthesizer is located in the ODU, it must withstand increased temperature and vibration. Also, the synthesizer control signals must be conveyed to the ODU. Furthermore, because the modulated IF signal from the IDU is conveyed as an analog waveform, it is subject to noise and other impairments, especially level variations, as it traverses the IFL cable.
Accordingly, there exists a need to improve upon the preciously discussed techniques to provide a VSAT in which: 1) no synthesizers are required in the ODU; 2) a narrowband IFL cable can be used; 3) sensitivities to the noise and level variation distortions on the IFL are eliminated; and 4) the overall complexities and cost of the VSAT are minimized.
The present invention provides a remote ground terminal designed to satisfy the aforementioned needs. Specifically, the invention includes an IDU which generates a fractionally modulated signal in a first frequency band having certain spectral purity, frequency resolution and stability characteristics, and an ODU which employs a straightforward multiplication method for translating the fractionally modulated signal to a minimum shift key (MSK) modulated signal in a second frequency band suitable for satellite transmission.
More particularly, the present invention comprises a ground terminal for transmitting satellite communications. In a preferred embodiment, the IDU utilizes microprocessor logic to control a DDS which serves as a carrier frequency source and a modulator. The fractional modulation riding on the DDS carrier signal appears as frequency shift key (FSK) modulation at {fraction (1/128)} of the final modulation which is ultimately transmitted to the satellite. The DDS signal is frequency translated to an intermediate frequency (IF) band and then fed through a cable multiplexor (MUX) to the ODU with nominal loss and disturbance.
In one embodiment of the present invention, the ODU includes a multiplying phase locked loop (PLL) and a frequency multiplier circuit for translating the signal received from the IDU to a MSK modulated signal suitable for satellite transmission. In another embodiment, however, the translation to a MSK modulated signal suitable for satellite transmission is performed employing only a multiplying PLL without the need for an additional multiplier. In either of these embodiments, the ODU acts as a frequency multiplier. The RF output of the ODU is a frequency which is an exact ratio of the IF signal frequency received from the IDU (in the presently preferred embodiments the ratio is x128). As noted earlier, the IF signal is frequency modulated with a deviation of {fraction (1/128)} of the desired modulation before it is conveyed to the ODU. The RF output at the ODU is then in modulated as desired. Because the RF signal is generated locally in the ODU in a PLL, its output level can be easily maintained at a constant level. This feature eliminates the need for complex power control circuits and eliminates the adverse effects of signal level variation caused by losses in the IFL cable. Because the function of both of these two ODU designs is equivalent from a performance point of view, either embodiment attains the advantages of the invention described herein.
The MSK modulated signal is then amplified by a solid state power amplifier (SSPA) before being fed to an ortho mode transducer (OMT). The OMT launches the signal into a feed horn, which in turn sends the signal to the small aperture antenna for transmission to the satellite.
According to the modulation scheme of the present invention, the ODU is simplified in that (1) no synthesizer is required, only a straightforward PLL or frequency multiplier is used; (2) power level control of the ODU""s output can be reduced or eliminated because cable impairments do not affect the signal; and (3) control circuits in the ODU are reduced because the modulating data is impressed upon the IF signal directly and no separate multiplexing or other signal conveyance means is required.
Further simplification is achieved in the IDU by combining the frequency reference and the modulator into one DDS circuit.
The invention itself, together with further objects and attendant advantages, will be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.